Video-telephone cameras have substantial capacity to capture the illumination over a number of pixel sites, and to divide the analog voltage representing the illumination amplitude at each pixel into a multiplicity of values represented digitally by a number of bits. They can also produce a large number of frames representing each image captured. The total number of bits per unit time then equals at least to the total number of bits per pixel times the number of pixels per frame times the number of frames per unit time.
The degree to which the amplitude of the analog signal appearing at each pixel site may be subdivided determines the pixel definition. For example, the amplitude may be divided into 128 values with 7 bits. This produces a fine gray scale gradation which a viewer may perceive as high definition and "fine" focus. On the other hand, dividing the analog amplitude at a pixel site into only two values with a single on off bit produces a high contrast that may be perceived as poor or "soft" focus.
Similarly, the frame rate can also be changed. A high frame rate assures smooth motion while a low frame rate may produce flicker or perception of jerky motion.
The capacity of a video-telephone camera to generate high bit rates from the product of the number of bits per pixel site, the number of pixels per frame, and the frame rate, exceeds the capacity of most transmission channels to carry the bit rate. Accordingly, a coder is used to reduce the number of pixel levels which the amplitude at any pixel site is divided, i.e. the number of bits per pixel sites, and/or the frame rate. The number of bits per pixel site is generally referred to herein as the pixel definition or focus. The coder selects a suitable pixel definition or focus relative to a particular frame rate on the basis of the channel capacity.
A typical coder operates according to a conventional CCITT standard. In that environment, the coder uses a predetermined relationship between pixel definition and frame rate. It is possible for the camera user to preset the relationship between the pixel definition or focus and the frame rate on the basis of the user's perception. However, such arrangement cannot accommodate rapid changes in scenes being taken without overloading the channel capacity or producing undesirable picture quality.